Such internal fixation devices allow for fixating the fractured joint or bone parts in their correct position and thereby promote healing of the fracture. In order to keep the damage to surrounding tissues as low as possible, less invasive methods of fracture stabilization are now more popular than ever and the development and application of minimally invasive and percutaneous fixation techniques have seen a great deal of interest in recent years.
The same revolution is taking place in pelvic fracture management. The sacro-iliac (SI) articulation is one of the most complex joints orthopedic surgeons to deal with since the anatomy of the sacrum and surrounding structures make diagnosis and fixation difficult.
One way to fix injuries of the pelvic ring is described in U.S. Pat. No. 5,601,550, for instance, which relates to a pelvic drill guide apparatus that is used to insert external fixation pins into a patient's pelvis that is fractured in “open book” fracture fashion. Multiple fixation pins are placed into the pelvis on each side and an external fixation frame holds the pins, and, thus the pieces of the pelvis together. The primary disadvantages of such external fixation of pelvic fractures include high risk of pin tract infections and highly impaired motion range of the patient due to the external frame.
Whereas open treatment of pelvic fractures as addressed in U.S. Pat. No. 5,601,550 predominated just a few years ago, today, open methods are giving way to newer techniques of minimal invasive procedures, e.g. closed or limited open reduction, coupled with percutaneous stabilization.
Internal bone fixation means are described in U.S. Pat. No. 2,489,868, for instance. The document relates to a bone fastening device for rigidly and fixedly fastening bone parts together. However, although numerous different approaches for internal bone fixation have been developed, choices of suitable screws and surgical tools for use in treatment of pelvic ring fractures are still very limited.
One complication associated with internal bone fixation, and in particular joint fixation, is migration of the screw over time owing to normal physiologic movement and remodeling of the bone. Depending on the bone density and structure, the screws either tend to migrate out of the bone, leading to loosening of the screw, or to migrate into the bone, which significantly complicates removal of the screws after the fracture has healed. The risk of migration of bone screws into the bone can be reduced by the use of washers which provide an increased retention surface and can further provide a compressive force on the bone region. However, the benefits provided by the washer are partly offset by the difficulty to remove the washer together with the screw later-on after the injury has healed. In the worst case, the washer forms a strong attachment to the bone, whereas the screw loosens and migrates out of the bore. This not only impairs the fixation of the fracture but also complicates removal of both, the migrated screw and the attached washer from the body.
To address this problem, fixed screw-washer assemblies have been described in the art. WO 2007/109140, for instance, relates to a screw assembly for insertion into a bone region. The screw comprises a proximal threaded area, which engages with a threaded area of the washer to fixedly secure the washer to the screw body. One drawback of such assemblies is that due to the increased friction surface provided by the washer, a greater amount of torque is required to be applied to the screw during fastening, which reduces the ease of operation and can damage the screw.